What's your typing speed? 15 seconds to invoke five functions is easy with little practice. Users who have played >1000 clashes probably have seen most of the ones in the pool and have already completed the clash before they've already seen it, just by looking at the example would already be enough to recall the logic.
Not to mention that most of the clashes are either trivial or ridiculously convoluted, underspecified - and also poorly tested for that there's not much point trying to cover edge cases, so sometimes people would submit code that doesn't even pass the sample tests and still get 100% if they know such a clash has that vulnerability. And there's a lot of them in that category.
Honestly, if you want proper speedcoding experience, you should use a site tailored for that; the pool for clash of code problems is quite small and repeats itself regularly, and they're also not tailored for correctness. Shortest is just code golf, except the site's language choices are so small that there are only a few competitive options (ruby, perl, bash...), and reverse mode are black box problems where if you're seeing it for the first time and having to figure it out, you've already lost to people who've already seen it at least five times before.
Cheating by copy-pasting for the 15-seconds range is preposterous - these are usually the people you'd see who are actually competent in speedcoding, and at the sub-20 seconds range it takes more time copy-pasting than actually solving a problem yourself. Cheaters using primitive techniques usually cash in at 20-25 seconds consistently, and it's very easy to recognize them because their performance is consistent with almost no deviation and they don't share their code. (This knowledge is based on my experience with dealing with cheat detection on an internet chess server)
As for what you can do about them, there's not much really, I've reported many cheaters and codingame has done nothing about them. Remember that this site is a recruitment tool, the administrators don't really care about your experience more so than they can keep their sponsors happy.